Genex: Enable COMPILE_LANGUAGE for COMPILE_DEFINITIONS with VS and Xcode

The set of compile flags used for a target's C and C++ sources is based
on the linker language.  By default this is always the C++ flags if any
C++ sources appear in the target, and otherwise the C flags.  Therefore
we can define the `COMPILE_LANGUAGE` generator expression in
`COMPILE_DEFINITIONS` to match the selected language.

This is not exactly the same as for other generators, but is the best VS
and Xcode can do.  It is also sufficient for many use cases since the
set of definitions for C and C++ is frequently similar but may be
distinct from those for other languages like CUDA.

Issue: #17435

1 files changed, 16 insertions, 11 deletions

diff --git a/Help/manual/cmake-generator-expressions.7.rst b/Help/manual/cmake-generator-expressions.7.rst
index dc5621a..85130a0 100644
--- a/Help/manual/cmake-generator-expressions.7.rst
+++ b/Help/manual/cmake-generator-expressions.7.rst
@@ -97,10 +97,9 @@ Available logical expressions are:
   compile features and a list of supported compilers.
 ``$<COMPILE_LANGUAGE:lang>``
   ``1`` when the language used for compilation unit matches ``lang``,
-  otherwise ``0``.  This expression may be used to specify compile options for
-  source files of a particular language in a target. For example, to specify
-  the use of the ``-fno-exceptions`` compile option (compiler id checks
-  elided):
+  otherwise ``0``.  This expression may be used to specify compile options
+  and compile definitions for source files of a
+  particular language in a target. For example:
 
   .. code-block:: cmake
 
@@ -108,10 +107,20 @@ Available logical expressions are:
     target_compile_options(myapp
       PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
     )
+    target_compile_definitions(myapp
+      PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX>
+    )
+
+  This specifies the use of the ``-fno-exceptions`` compile option
+  and ``COMPILING_CXX`` compile definition for C++ only
+  (compiler id checks elided).
 
-  Note that with :ref:`Visual Studio Generators` there is no way to represent
+  Note that with :ref:`Visual Studio Generators` and :generator:`Xcode` there
+  is no way to represent target-wide compile definitions separately for
+  ``C`` and ``CXX`` languages.
+  Also, with :ref:`Visual Studio Generators` there is no way to represent
   target-wide flags separately for ``C`` and ``CXX`` languages.  Under these
-  generators, target-wide flags for both C and C++ sources will be evaluated
+  generators, expressions for both C and C++ sources will be evaluated
   using ``CXX`` if there are any C++ sources and otherwise using ``C``.
   A workaround is to create separate libraries for each source file language
   instead:
@@ -125,15 +134,11 @@ Available logical expressions are:
     target_link_libraries(myapp myapp_c myapp_cxx)
 
   The ``Makefile`` and ``Ninja`` based generators can also use this
-  expression to specify compile-language specific compile definitions
-  and include directories:
+  expression to specify compile-language specific include directories:
 
   .. code-block:: cmake
 
     add_executable(myapp main.cpp foo.c bar.cpp)
-    target_compile_definitions(myapp
-      PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX>
-    )
     target_include_directories(myapp
       PRIVATE $<$<COMPILE_LANGUAGE:CXX>:/opt/foo/cxx_headers>
     )